Pros
I had the pleasure of working at Kumu for a 6 month internship and it was an amazing and rewarding experience with as much responsibility as you aspire to. The technology is cutting edge, the people are brilliant, and the environment is fun and full of perks (breakfast, lunch, snacks, beer, outings, BBQs, dogs!). Loved my time in the South Bay area and with all my colleagues at Kumu.
Cons
Some days are really long but that's all part of working at a startup, and that's part of the fulfillment is working hard and having real impact. No serious cons, except maybe some of the unreliable prototype 3rd party technology used in the lab.